The present type torch uses an electric arc struck between a pair of electrodes to heat a working gas. The gas extends the arc and it is heated by the arc such that it becomes ionized and disassociated to form a plasma. Such torches can usually operate in a so-called transferred mode wherein the arc and plasma jet extend from a nozzle to the workpiece being heated and in some cases the torches operate in a so-called non-transferred mode in which case the arc impinges the wall of the nozzle which functions as an anode and only the plasma effluent is projected as a jet beyond the nozzle toward the workpiece. The basic operation of torches of this type are described in detail in U.S. Pat. No. 2,960,594. Generally, they are used in applications requiring intense heat such as in continuous casting, melting, sintering, and like processes.
Over the years since the above basic patent issued, various improvements have been made to plasma jet torches to increase their power, efficiency and the operating life of their parts. For example, U.S. Pat. No. 3,027,446 describes an electric arc torch in which the plasma-forming gas is introduced into the torch through a relatively few tangentially disposed small holes to create a vortex which surrounds the electric arc. This gas swirl stabilizes the arc and cools the wall of the nozzle through which the plasma projects. U.S. Pat. No. 3,118,046 discloses a plasma jet torch whose cathode element is located at the very bottom of a well to lengthen and stabilize the arc, while minimizing erosion of that element due to reaction with the working gas. U.S. Pat. No. 3,297,899 discloses a similar torch having a wasp-waisted or constricted anode nozzle through which the arc passes in order to maintain a relatively high working gas pressure in the torch so that the torch can deliver a jet flame of high power, but low pressure at a reasonable current level.
Invariably, such torches have certain requirements with respect to the electric power supplied to the torch and the flow rate through the torch of the plasma-forming gas if the torch is to operate in a reliable and stable manner. If the power to the torch is too low, there will be insufficient ionization of the gas to form a useful plasma. If the gas velocity in the arc pathway is insufficient, the arc will be unstable and flashback or premature arcing to the electrode wall will occur. On the other hand, the upper limit of the power that may be supplied to the torch depends primarily upon the structural limitations of the torch components. For example, if there is too much power to the torch, pitting and even melting of its electrodes can result and, if the gas velocity becomes too high, erosion of the nozzle electrode can occur or the arc may be blown out. Present day plasma jet torches including those described in the aforesaid patents are disadvantaged in that their regions of stable operation within the aforesaid limits are rather small. Apparently, the arc wanders somewhat in its passageway due to small moments of its electron emission site and to small variations or pulsations in the working gas vortex that supports the arc. Resultantly, particularly at high power levels, arc fingers tend to strike prematurely to the electrode walls causing unstable operation and temperature variations in the plasma effluent, as well as electrode pitting and erosion of the electrodes. Power delivered to the plasma developed by the torch is the power supplied less electrode and radiation losses which appear as heating of the cooling water supplied to the torch. Consequently, the realized power of a given torch can only be varied over a relatively small range. As a result, arc torches have to be designed specifically for operation in a selected rather narrow power range. For example, a torch designed to operate at relatively low power, e.g. 30 to 50 KW, to heat a small kiln in a laboratory cannot be operated at higher power levels, e.g. 120 to 130 KW, to heat a scaled-up version of the kiln in a pilot plant. Neither will a torch designed to operate at a high power level work efficiently at low power. Therefore, a particular installation may be required to stock several different torches in order to satisfy all of its heating requirements.
Also, some conventional torches are not particularly efficient even within their designed operating range. The efficiency of a torch is measured by the power delivered to the plasma with relation to the amount of power supplied to the torch, the difference being electrode and radiation losses reflected as heating of the cooling water supplied to the torch. It is not uncommon for some conventional torches to operate at an efficiency as low as 50% so that the cost of using those torches is quite high. Also, in many present day electric arc torches, fairly rapid deterioration of the torch parts, particularly their electrodes, occurs over time because their arcs become unstable and tend to wander causing overheating, erosion and pitting of those parts as noted above. Such damage to the electrodes further destabilizes the arc resulting in more erosion and damage to the torch parts. Accordingly, those torches suffer from excessive parts losses and downtime for repair and maintenance.